1. Field of the Invention
The present invention relates to sensors, or more generally to apparatuses for exciting and detecting mechanical deflections and to the use of such apparatuses for mass-sensing, scanning probe microscopy, filtering electronic circuits, and similar applications. For such applications, cantilever structures and transducers of this structure are widely used because of their relative simplicity and robustness.
2. Related Art
As mentioned, cantilever transducers have a number of applications as force sensors for scanning probe microscopes and as acceleration sensors; other applications include their use as chemical and biochemical sensors. An examples is shown in Thundat et al U.S. Pat. No. 5,719,324 xe2x80x9cMicrocantilever Sensorxe2x80x9d. Typically, cantilever devices employ piezoelectric films as one layer of the cantilever, but capacitive excitation has also been demonstrated, cf. S. Suzuki et al: xe2x80x9cSemiconductor capacitance-type accelerometer with PWM electrostatic servo techniquexe2x80x9d in Sens. Actuators A21-A23, pp. 316-319, 1990. Usually the cantilever is fabricated using micromachining processes on a silicon wafer, cf. S. M. Sze in the book xe2x80x9cSemiconductor Sensorsxe2x80x9d by John Wiley and Sons, Inc., New York.
However, the known devices and structures have some disadvantages which limit their applicability, as will be explained. Thermal actuation of the cantilever, for example, has the disadvantage of requiring a relatively high heating power to excite cantilevers at high frequencies. Furthermore, the application of heat to a chemically sensitive cantilever may clash with the desired functionality of the device. On the other hand, electrostatic actuationxe2x80x94which allows for high excitation frequenciesxe2x80x94requires application of an electrostatic field by a counter electrode, which complicates structure and fabrication and thus limits the applicability of such a cantilever. Piezoelectric actuation requires the use of piezoelectric materials on the cantilever which often interfere with other surface materials, as e.g. required for chemical sensors. Also, it would be advantageous to allow the integration of a cantilever with the appropriate electronic sensing and/or excitation circuits. This limits the applicable fabrication processes.
The above problems are often aggravated when a resonant cantilever is used, i.e. a cantilever oscillating with its resonant frequency, where usually the frequency variation affected or influenced by external matters is used as readout. As an example, thermal excitation with a purely sinusoidal heating voltage may result in smaller temperature increases. However, the deflection of the cantilever will be proportional to the heating power and, thus, at twice the frequency. The operation of such a device in oscillation mode requires signal processing of advanced complexity.
For some other applications, magnetic actuation of cantilevers has been proposed. B. Shen et al describe such a structure in xe2x80x9cCMOS Micromachined Cantilever-in-Cantilever Devices with Magnetic Actuationxe2x80x9d, IEEE Electron. Device Letters, Vol. 17, No. 7, July 1996, pp. 372-374. The structure described by Shen, however, is a rather complex microactuator specifically designed to achieve a large angular deflection. It consists so-to-speak of conductor system carrying a conductor-free central cantilever. A current in the conductor system and a surrounding static magnetic field produce Lorentz forces deflecting the central cantilever. The authors concentrate on the fabrication of this structure to deliver high angular deflections which are detected using external optical components. However, from a mechanical and electrical viewpoint, just this complexity limits the applicability of the structurexe2x80x94it increases system cost and poses limits on the miniaturization.
Lee et al. describe a cantilever for application in the so-called tapping mode in scanning probe microscopy in xe2x80x9cCantilever with integrated resonator for application of scanning probe microscopexe2x80x9d, Sensor and Actuators 83 (2000), 1996, pp. 11-16. The cantilever uses a magnetically driven torsional resonator to monitor the interaction force between the cantilever tip and the sample to be probed. In close vicinity to the sample, the vibration amplitude of the torsional resonator structure is a function of the force (gradient) between sample and tip. The vibration amplitude is measured using a second inductive coil on the resonator. The cantilever is then moved by magnetic actuation so that the vibration amplitude of the resonator remains constant. This way, the cantilever follows the topography of a sample. However, with this method, cantilever deflections are only detected in the close vicinity of a sample and thus its applicability is limited to the case of dynamic scanning probe microscopy.
Starting from the described prior art, it is a primary object of the invention to provide a simple and robust cantilever device requiring neither piezoelectric material nor thermal nor capacitive actuation and using magnetic actuation instead.
Another object of the invention is the provision of a cantilever apparatus highly compatible with microelectronic circuits to enable simple fabrication and possible integration of cantilever and associated microelectronics.
A further object is to provide a robust cantilever device whose excitation mechanism does not interfere with chemically sensitive films on the cantilever.
The present invention circumnavigates the above problems and provides a solution to the above objects issues by creating a novel magnetic excitation approach for the cantilever. The gist of the invention can be said to consist in the integration of part of the magnetic excitation structure into the cantilever.
Thus, the invention can be briefly described as a cantilever structure and device comprising: a conductor on or in a cantilever, a static magnetic field orthogonal to said conductor, and the energization of the conductor with an electric current to achieve the desired function. In a specific embodiment, the conductor is formed as a current loop. Another embodiment shows a straight conductor, but other forms are also possible. Further, excitation may be in the resonant mode or non-resonant mode.
Thus, a cantilever structure and device according to the invention neither requires piezoelectric material nor any thermal actuation. It provides for a robust and simple cantilever structure that is highly compatible with microelectronic circuits, so that its direct integration into microelectronic circuits represents no problem.
A further advantage of the present invention is its non-interference with chemically sensitive films, so that highly sensitive chemical sensors can be implemented.
Yet another advantage of the present invention is its ready adaptability as a resonant mass-sensitive device. This is because the resonance frequency of a cantilever depends on the mass load. Magnetic actuation here offers the important advantage of low power consumption.
A still further advantage is that separate conductors or current loops can easily be provided on the cantilever, one e.g. for excitation, another for readout.
With all these advantages, a cantilever device according to the invention may be used in scanning probe microscopy, but is also easily adaptable as an acceleration sensor, as filtering device in electronic circuits, or for any other purpose where resonant or non-resonant cantilever structures are desired.